Thermal type flow sensors for measuring flow rate or flow velocity of a fluid come in two types. Specifically, there have been known a type in which a shift in spatial temperature distribution of a fluid formed by heat generation of a heater, which is a heating element, is produced via the flow of fluid, and this shift is detected by a temperature sensor (indirectly heated type) and a type in which a change in electric power or a change in resistance caused by taking away the heat of a heating element by a fluid is detected, by which flow velocity or flow rate is detected (self-heating type). For the former conventional flow sensor, a type in which a sensor is patterned on the surface of silicon, and a fluid under measure is allowed to flow directly to this sensor pattern is available. However, a sensor chip utilizing silicon has a drawback of being liable to be corroded by a corrosive gas though it is excellent in terms of sensitivity and response. Therefore, in the case where the fluid under measure is a gas, only a gas that chemically eats away silicon can be allowed to flow.
However, in recent years, in addition to the flow sensor used for noncorrosive gases only, an indirectly heated type flow sensor having a construction usable for a liquid or a corrosive gas has come to be used (for example, refer to Unexamined Japanese Patent Publication No, 2002-122454, which is Patent Document 1).
Such a flow sensor 3 has, as shown in FIG. 4, a construction including a substrate 310 the surface side of which faces to a flow path 201 for a fluid under measure and a flow path forming member 220 and a plate 230, which are disposed so as to face to each other with the substrate 310 being interposed therebetween. The substrate 310 is made of stainless steel, and is formed into a plate shape having a thickness of about 50 to 150 μm. On the surface of the substrate 310 opposite to the flow path side, an electrical insulating film is formed, and on this film, a temperature detecting means for measuring flow velocity (flow rate) of a fluid, an ambient temperature sensor, electrode pads, and a wiring metal thin films are formed. Thus, a thin stainless steel sheet is used as the substrate 310, and the side opposite to the sensor forming surface is used as a flow path, by which this flow sensor 3 can be used in the case where the fluid under measure is a corrosive fluid.
Such an electrode insulating film of stainless steel sheet is formed by a silicon oxide (SiO2) film, silicon nitride film, alumina, polyimide film, or the like having a thickness of, for example, several thousand angstroms to several micrometers. The silicon oxide film can be formed by, for example, sputtering, CVD, or SOG (spin-on-glass). On the other hand, the silicon nitride film can be formed by sputtering, CVD, or the like.
Also, on the surface of this electrical insulating film, a flow velocity detecting means and an ambient temperature detecting means individually including a plurality of electrode pads and a wiring metal thin film are formed by a publicly known thin film forming technique. Specifically, the flow velocity detecting means and the ambient temperature detecting means are formed by depositing a material such as platinum on the surface of electrical insulating film and by etching the material into a predetermined pattern. The flow velocity detecting means and the ambient temperature detecting means are individually connected electrically to the electrode pad via the wiring metal thin film. Further, each of the electrode pads is connected to an electrode terminal of a printed wiring board provided, via a spacer, above the sensor chip via a bonding wire, not shown.
Patent Document 1: Unexamined Japanese Patent Publication No, 2002-122454
In the case where the sensor chip is formed of an anticorrosion metal such as stainless steel, hastelloy, or inconel, it is necessary to finish the surface on which the insulating film (oxide film) is formed so as to be flat by polishing. However, such a metal does not consist of single crystals unlike a semiconductor. That is to say, silicon consists of single crystals and the surface thereof is strictly flush. Since stainless steel is a metal, a slight flaw is inevitably produced on the surface by polishing. No matter how carefully surface finishing by polishing is performed, surface roughness of some degree is produced on the surface.
Considering very fine irregularities constituting the surface roughness of some degree of the stainless steel surface, it is necessary to form an insulating film having a thickness of 10 μm or larger to secure insulation between a diaphragm and the sensor.
In the case where a diaphragm having a stainless steel base, in which an insulating film having the above-described thickness is formed on the surface thereof and thereby a strain gage is formed on the insulating film, is used as a pressure sensor diaphragm, such a sensor diaphragm especially has no problem associated with heat transfer characteristics. Therefore, such a metallic diaphragm can be used as a detecting element of a pressure sensor. However, in the case where a diaphragm provided with an oxide film and a sensor device having the same construction as that of the above-described diaphragm is used for a flow sensor, thermal characteristics such as heat conductivity degrade, so that actual use of such a diaphragm for a flow sensor is unfavorable.
Specifically, if such a diaphragm construction of pressure sensor is diverted to a thermal type flow sensor, the insulating film with low heat conductivity exerts an adverse influence on the sensitivity and response of the sensor. Silicon oxide (SiO2), which is generally used as an insulating film, has a heat conductivity of 1.4 [W/mk], and austenitic stainless steel has a heat conductivity of 16 [W/mk]. Since the thickness of insulating film of 1 μm corresponds to the thickness of austenitic stainless steel of about 11 μm, when the thickness of insulating film increases, heat transfer due to thermal conduction between the sensor device and the fluid under measure via the thickness portion of diaphragm and the thickness portion of insulating film is hindered extremely.
On the other hand, the linear expansion coefficients of martensitic stainless steel, austenitic stainless steel, and silicon oxide used as the insulating film are as follows:    (1) Linear expansion coefficient of martensitic stainless steel (SUS400 series) . . . approximately 10×10−6/° C.    (2) Linear expansion coefficient of austenitic stainless steel (SUS300 series) . . . approximately 16×10−6/° C.    (3) Linear expansion coefficient of silicon oxide . . . approximately 0.8×10−6/° C.
As can be seen from such a difference in linear expansion coefficient between martensitic stainless steel and austenitic stainless steel, in the case where a single insulating film is formed on the surface of stainless steel, the stainless steel capable of being used as the diaphragm to relieve thermal stress due to the rising or lowering of temperature is limited to martensitic stainless steel, so that the degree of freedom in selecting material is restricted. That is to say, inherently, it is preferable that an insulating film consisting of, for example, silicon oxide be formed on the surface of austenitic stainless steel having higher corrosion resistance. However, such a configuration has a drawback in that, for example, a crack is formed in the insulating film by thermal stress because of the above-described difference in linear expansion coefficient between austenitic stainless steel and silicon oxide. Therefore, in the present situation, austenitic stainless steel cannot be used for the metallic diaphragm of flow sensor.
As described above, regarding the flow sensor having a construction in which a sensor pattern is formed, via an insulating film, on one surface of a diaphragm consisting of an anticorrosion metal such as stainless steel, and the other surface is brought into contact with a fluid under measure, in the case where only one layer of insulating film is present, an insulating film thickness that can withstand a withstand voltage of 100 V or higher is required, and on the other hand, in terms of flow sensor characteristics, the insulating film thickness is required to be as small as possible to improve heat conduction. Such contradictory requirements must be satisfied. In a configuration in which only one layer of insulating film with a thickness that satisfies both of the requirements is formed, the difference in thermal expansion coefficient between the insulating film and the stainless steel substrate cannot be relaxed, and thus there arises a problem in that a crack is formed in the insulating film in a high-temperature environment, and hence the sensor pattern on the insulating film is broken.
In the case where the diaphragm is formed of an anticorrosion metallic material such as stainless steel, the insulating film with a thickness of about 10 μm, which is indispensable in this case, becomes hindrance to the production of a thermal type flow sensor having high sensitivity and response.
An object of the present invention is to provide a flow sensor which has improved sensitivity and response and is especially suitable for measuring the flow rate of a corrosive fluid.